JP2000067858A - Electrode for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery having a high capacity and a stable cycle characteristic by forming an electrode of a molded body containing a powder of a predetermined lithium-containing compound nitride, a conductive agent and a binder and specifying an average particle diameter of the lithium-containing compound nitride. SOLUTION: An electrode is formed of a molded body containing a lithium- containing compound nitride powder represented by a formula: LiαMβN (M preferably represents at least one transition element selected from the group consisting of cobalt, iron, manganese, copper and nickel, 0<=α<=3.0, 0.1<=β<=0.8), a conductive agent and a binder as an electrode for a lithium secondary battery. Then it is preferable that an average particle diameter of the lithium-containing compound nitride powder used is in a range of 20 μm or less and 1 μm or more. The lithium-containing compound nitride having a performance reversibly charging/discharging lithium is used for an electrode and is used by finely pulverizing a particle diameter of the powder from the first.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery, and more particularly to an electrode using a lithium-containing composite nitride as an active material.

[0002]

2. Description of the Related Art In recent years, high energy density lithium ion batteries using lithium cobalt oxide for the positive electrode and carbon for the negative electrode have been widely used as practical batteries. These batteries are
2. Description of the Related Art As electronic devices become ultra-compact and lightweight, higher energy density is desired. In recent years, application of a nitrogen compound containing lithium, that is, a lithium-containing composite nitride to a lithium battery has been attempted. For example, those using lithium-containing composite nitride as an electrode material of an electrochemical element including a lithium secondary battery (Japanese Patent Laid-Open No. 7-78609), those using a mixture of lithium-containing composite nitride and metal Li ( Japanese Patent Application Laid-Open No. 7-320720) and those using an amorphous lithium-containing composite nitride (Japanese Patent Application Laid-Open No. 7-19258).
No. 5). The working electrode potential of this lithium-containing composite nitride is around 0 to 2 V on the basis of the Li potential, and its use as a negative electrode material is also considered.

The charge / discharge mechanism of lithium-containing composite nitride is as follows:
Basically, it is considered to be an intercalation / deintercalation reaction of Li in the lithium-containing composite nitride. The reversible capacity when used as a negative electrode material is extremely large, and a battery with a high energy density can be realized. As a lithium-containing composite nitride, for example, Li _3-x C
If o _x N of Co substitution amount x of the material of x = 0.3 to 0.4, an average operating potential of this compound is 0.7V Atari in potential reference of Li, the reversible capacity is 700~800m
Ah / g is obtained. Furthermore, the true specific gravity of this material is about 2.0 g / cc, which is equivalent to graphite. On the other hand, in a normal lithium ion battery using lithium cobalt oxide (LiCoO ₂ ) for the positive electrode and carbon for the negative electrode, the average operating potential of graphite used for the negative electrode is 0.1% with respect to the potential of Li. ０．２0.2 V and its theoretical capacity is 370 mAh / g. Therefore, in a battery using lithium cobalt oxide for the positive electrode and a lithium-containing composite nitride for the negative electrode, the battery voltage is reduced by about 0.5 V, but the battery capacity can be dramatically improved, and Can have a very high energy density.

[0004]

However, when a lithium secondary battery is formed using a lithium _- containing composite nitride such as Li _3-x Co _x N for the negative electrode and LiCoO ₂ for the positive electrode, the capacity is extremely large. It turned out that when the battery was actually constructed and charged and discharged, the charge / discharge capacity gradually decreased with the charge / discharge cycle, and the cycle characteristics deteriorated significantly. The present invention solves the above problems,
An object of the present invention is to provide an electrode that provides a lithium secondary battery having high capacity and stable cycle characteristics.

[0005]

In order to solve the above-mentioned problems, the present invention uses a lithium-containing composite nitride having a reduced particle size. That is, the electrode for a lithium secondary battery of the present invention is a lithium-containing composite nitride represented by a general formula LiαMβN (M represents a transition element, 0 ≦ α ≦ 3.0, 0.1 ≦ β ≦ 0.8). And a lithium-containing composite nitride powder having an average particle size of 20 μm or less.
Here, M is preferably at least one element selected from the group consisting of cobalt, iron, manganese, copper and nickel.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION A lithium-containing composite nitride is generally represented by the formula LiαMβN, and the performance as an electrode active material greatly changes depending on the type of the element M and the values of α and β. In particular, a compound using a transition element for M and having a numerical range of α and β of 0 ≦ α ≦ 3.0 and 0.1 ≦ β ≦ 0.8 has the ability to reversibly charge and discharge lithium, that is, It has excellent performance as an electrode active material for lithium secondary batteries. Among them, those using at least one selected from the group consisting of cobalt, iron, manganese, copper, and nickel as the transition element M are a high-capacity electrode active material. The present invention is intended to improve the charge / discharge cycle characteristics of a battery using such a high capacity lithium-containing composite nitride as an electrode active material.

The present invention is based on the finding of the following as the cause of the deterioration of the cycle characteristics, which is the above-mentioned problem. When a lithium-containing composite nitride is synthesized,
Obtained as hexagonal crystalline. When a battery is formed using this lithium-containing composite nitride as a negative electrode active material and charged and discharged, the crystal state of the lithium-containing composite nitride changes significantly. That is, by the first charge / discharge, the hexagonal lithium-containing composite nitride changes to amorphous, and the miniaturization proceeds. As a result, the current collection between the active material and the conductive agent is hindered, and the cycle characteristics deteriorate. Further, in the lithium-containing composite nitride, the cycle proceeds while the active material repeatedly undergoes large expansion and contraction in an amorphous state. This further deteriorates the electrical contact of the lithium-containing composite nitride with the conductive agent.

In particular, when the particle size of the active material is relatively large, the cycle characteristics tend to deteriorate significantly. This is because, as the particle size of the active material increases, the number of grain boundaries generated in the active material particles increases due to the miniaturization, which causes inhibition of lithium ion transfer and interruption of electron conduction. In addition, as the particle size of the active material increases, the number of interfaces through which the electrolyte penetrates into the voids formed by the miniaturization increases, and as a result, the expansion and contraction width also increases. It has been found that the electrical contact between the active material particles and the conductive agent (such as carbon powder) particles is loosened and causes a decrease in current collection efficiency. In order to prevent this and configure a battery having excellent charge / discharge cycle characteristics, it is necessary to use a lithium-containing composite nitride having a fine particle diameter from the beginning, and in particular, to use an average lithium-containing composite nitride powder. It is preferable that the particle size is 20 μm or less and 1 μm or more. Here, when a lithium-containing composite nitride having an average particle diameter of 1 μm or less is used, the mixture density of the electrode is low, the powder becomes a so-called bulky powder, the filling amount of the active material in the negative electrode is reduced, and suitable selection and Not be.

[0009]

The present invention will be described in more detail with reference to the following examples. FIG. 1 is a cross-sectional view of a button-type battery manufactured as a test cell for comparatively examining the cycle characteristics of a lithium secondary battery according to the present invention. In FIG. 1, reference numeral 1 denotes a sealing plate made of stainless steel. A nickel mesh 2 is fixed to the inner surface of the sealing plate 1 by resistance welding. The negative electrode 3 containing the lithium-containing composite nitride is formed on the copper foil 4, and current is collected by pressing the copper foil 4 on the nickel mesh 2. The positive electrode 7 using LiCoO ₂ as an active material is formed on an aluminum foil 8 and then punched into a disk having a diameter of 15 mm, and is electrochemically formed in advance to be in a charged state. A stainless steel mesh 10 is fixed to the inner surface of the positive electrode case 9 made of stainless steel by resistance welding. The positive electrode 7 is collected by pressing an aluminum foil 8 onto a stainless steel mesh 10 in a positive electrode case 9. The positive electrode 7 is arranged on the inner surface of the case 9, the porous membrane separator 6 made of polyethylene is placed on the positive electrode 7, and after injecting the electrolyte, the sealing plate 1 to which the negative electrode 3 and the gasket 5 are attached is combined with the case. The part is sealed by caulking and the sealed battery is assembled. The organic electrolyte is obtained by dissolving 1 mol / liter of LiPF ₆ in a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1.

In the above battery, a negative electrode containing a lithium-containing composite nitride was produced as follows. First, the composition formula at the time of synthesis as a lithium-containing composite nitride is Li _2.6 Co _0.4
An N active material was used. This lithium-containing composite nitride is
A predetermined amount of lithium nitride (Li ₃ N) and metallic cobalt powders were mixed and heat-treated at 700 ° C. for 8 hours in a nitrogen atmosphere. Synthesized Li _2.6 Co _0.4
N was obtained as a sintered body, which was ground in an agate mortar to obtain a powder. Preparation of samples having different particle diameters was performed by adjusting the pulverization time and classification by a sieve. In preparing an electrode for comparative study, a particle size distribution was measured to confirm the average particle size before use. With the above method, the average particle size of each is 3
0 μm, 25 μm, 20 μm, 15 μm, 10 μm, 5
Samples of μm, 1 μm, and 0.5 μm were prepared. At this time, the distribution width of the particle diameter of the sample used was adjusted so as to be within 50% of the average particle diameter ± the average particle diameter.

Next, the negative electrode is made of the Li
_2.6 An active material powder of Co _0.4 N, a carbon powder of a conductive agent, and styrene butadiene rubber (SBR) as a binder have a weight ratio of 8
The mixture was mixed at 0: 18: 2 and dispersed in dehydrated toluene to prepare a slurry. The slurry was applied to a negative electrode current collector made of a copper foil having a thickness of 18 μm using a doctor blade, and dried.
Rolled to form a negative electrode sheet. From this negative electrode sheet, a negative electrode plate having a diameter of 16 mm was punched to form a disk-shaped electrode. On the other hand, the positive electrode was prepared by mixing lithium cobalt oxide powder, carbon powder of a conductive agent, and polyvinylidene fluoride resin as a binder in a weight ratio of 85:
The mixture was mixed at 10: 5 and dispersed in dehydrated N-methylpyrrolidinodon to prepare a slurry. The slurry was applied to a positive electrode current collector made of an aluminum foil having a thickness of 20 μm using a doctor blade, dried, and then rolled. To form a positive electrode sheet. From this positive electrode sheet, a positive electrode plate having a diameter of 15 mm was punched to form a disk-shaped electrode. These positive electrodes were electrochemically formed in advance and charged to match the initial state of the negative electrode.

The battery fabricated as described above was charged at 2 mA.
The charge / discharge cycle in which the battery was charged to a cut-off voltage of 4.1 V with a constant current of 0.1 V and discharged to a cut-off voltage of 2.0 V with a constant current of 2 mA was repeated. FIG. 2 shows the charge / discharge cycle characteristics of each of the batteries using the active materials having the above-described various particle diameters. The change in the discharge capacity when charge / discharge is repeated up to 50 cycles under the above charge / discharge conditions is plotted. It was done. FIG.
As is clear from the above, the cycle characteristics of a battery using an active material having a large average particle size are greatly deteriorated, and particularly when the average particle size is 25 μm or more, the cycle characteristics of the battery are rapidly deteriorated. As described above, Li _2.6 Co _0.4 N
Has a mean particle size of 20 μm.
It is understood that the value is preferably equal to or less than m.

The present invention is intended to prevent the capacity from being deteriorated due to the charge / discharge cycle, and basically limits the upper limit of the particle diameter. That is, as can be seen from FIG. 2, the lower limit of the particle diameter does not impair the cycle characteristics by reducing the particle diameter. There is no lower limit on particle size for cycle reversibility. However, when the particle size is small, the powder becomes bulky and, for example, the rolling strength is kept constant (20%).
When the density of the mixture layer of the electrode plate manufactured at 0 kg / cm ² ) was measured, the density of the mixture layer was 1.4 to 1.6 g / cc when the average particle diameter was 1 μm or more. However, 1μ
m, for example, 0.5 μm, the mixture layer density decreased to 1.1 g / cc. As a result, the battery discharge capacity also became smaller as can be seen from FIG.

In the above embodiment, a battery using Li _2.6 Co _0.4 N as a negative electrode has been described, but the same test was performed for other lithium-containing composite nitrides. as a result,
General formula LiαMβN (M: transition element, 0 ≦ α ≦ 3.0,
0.1 ≦ β ≦ 0.8) and an active material in which M is at least one element selected from the group consisting of cobalt, iron, manganese, copper and nickel. Gave similar results.

[0015]

As described above, according to the present invention, an electrode which provides a lithium secondary battery having a high capacity and excellent cycle characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a button type battery according to an embodiment of the present invention.

FIG. 2 is a diagram showing cycle characteristics of a battery using various negative electrodes.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sealing plate 2 Nickel mesh 3 Negative electrode 4 Copper foil 5 Gasket 6 Separator 7 Positive electrode 8 Aluminum foil 9 Positive electrode case 10 Stainless steel mesh

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masaki Hasegawa 1-1, Matsushita-cho, Moriguchi-shi, Osaka Matsushita Battery Industry Co., Ltd. (72) Inventor Shuji Tsutsumi 1-1, Matsushita-cho, Moriguchi-shi, Osaka Matsushita Battery (72) Inventor: Shin Fujino 1-1, Matsushita-cho, Moriguchi-shi, Osaka Matsushita Battery Industrial Co., Ltd. (72) Shigeo Kondo 1-1-1, Matsushita-cho, Moriguchi-shi, Osaka Matsushita Battery Inside (72) Inventor Takahisa Masayo 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Japan Inside Telegraph and Telephone Corporation (72) Inventor Yoji Sakurai 3-192-2, Nishi-Shinjuku, Shinjuku-ku, Tokyo Japan F-term in Telegraph and Telephone Corporation (reference) 5H003 AA02 AA04 BB47 BD00 BD02 5H014 AA02 EE10 HH06 5H029 AJ03 AJ05 AK03 AL01 AM05 AM07 BJ03 HJ02 HJ05

Claims (2)

[Claims] 1. A powder of a lithium-containing composite nitride represented by a general formula LiαMβN (M represents a transition element, 0 ≦ α ≦ 3.0, 0.1 ≦ β ≦ 0.8), a conductive agent, and An electrode for a lithium secondary battery, comprising a molded body containing a binder, wherein the lithium-containing composite nitride powder has an average particle size of 20 μm or less. 2. The electrode for a lithium secondary battery according to claim 1, wherein M is at least one element selected from the group consisting of cobalt, iron, manganese, copper and nickel.